Subunits hexokinase

Steitz, T.A., et al. High resolution x-ray structure of yeast hexokinase, an allosteric protein exhibiting a non-symmetric arrangement of subunits. 

Hexokinase has a molecular weight of 102,000 and is composed of two identical subunits of 51,000 molecular weight each (20-22). In yeast, hexokinase exists as a mixture of two isoenzymes (23). These forms have been named A and B (23). These isoenzymes can be separated by chromatography and have been found to be chemically different (23, 24). Some of the earlier work with hexokinase was done with preparations that contained a mixture of isoenzymes. Work also was done with enzyme that was proteolytically modified. These variations undoubtedly have been responsible for some of the controversy concerning the properties of this enzyme. 

Binding studies have shown that hexokinase will bind one glucose molecule per subunit (63). Direct binding studies with ATP could not be determined since its binding constant is apparently too high (64). 

Work has progressed on three different crystal forms of hexokinase. These have been labeled BI, BII, and Bill. Form BI is in the space group P2j 22 j with 4 molecules per unit cell (68). An electron density map has been calculated to 6 A resolution. The two subunits of hexokinase in this crystal form are related by a rotation of 180° plus a translation of 3.5 A along the sym-... 

Fig. 16. A schematic drawing of the hexokinase dimer showing the location of observed glucose and nucleotide binding sites. The two sugar binding sites are represented by 5 and S. The intersubunit AMP-PNP site I is formed by regions Iu and Id of each subunit. The symmetry related sites are indicated by Iu and Id. The location of the AMP binding sites in the monomeric Bill crystal form are labeled A and A. From Anderson et at. (72).

As mentioned, AMP-PNP or ADP in the presence of glucose will bind only to the BII crystals at a site between the two subunits. Nucleotides bound at this site appear to be in a fully extended conformation (73). ATP analogs bound at this site make contact with amino acid residues from both subunits. The y-phosphate of ATP bound at this site is 20 A from the 6-hydroxyl of bound glucose on one subunit and 30 A from the glucose on the other subunit (73). It has been proposed that this site is an allosteric regulatory site for hexokinase and not the substrate site for ATP where phosphoryl transfer occurs (73). 

Identical subunits Glyceraldehyde-3-phosphate dehydrogenase, catalase, alcohol dehydrogenase, hexokinase Mostly intracellular enzymes rarely have disulfide bonds... 

Maity H, Maiti NC, Jarori GK. Time-resolved fluorescence of tryptophans in yeast hexokinase-PI effect of subunit dimerization and ligand binding. Journal of Photochemistry and Photobiology B 2000, 55, 20-26. 

The tertiary stmcture of a protein is the complete three-dimensional stmcture of the polypeptide chain. If multiple polypeptide chains are present, the arrangement of their polypeptide chains with respect to each other is the quaternary stmcture in such cases, enzymes are polymers conposed of two or more subunits (Table 3). In order to illustrate the tertiary stmcmre of a protein, the three-dimensional stmcture of the glycolytic enzyme hexokinase is shown in Fig. 3 with the aid of a space-filling model. Hexokinase catalyzes the phosphorylation of D-glucose with ATP. The groove in the middle of the stmcmre is where the substrates bind (Cantor Schimmel, 1980). 

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